Method of sterilizing

ABSTRACT

THE STERILIZING PROCESS IS CARRIED OUT IN A SCALED CONTAINER OR AUTOCLAVE WHEREIN AIR IN THE CHAMBER IS DISPLACED WITH STEAM WHILE THE CHAMBER IS MAINTAINED AT VACUUM CONDITION. THE STEAM IS FED IN AT A RATE NO GREATER THAN THE CAPACITY OF THE VACUUM PUMP SO THAT THE PUMP WILL MAINTAIN THE DESIRED PRESSURE IN THE CHAMBER. STEAM THEREBY HEATS THE LOAD TO THE DESIRED TEMPERATURE IN A SHORT TIME AND THEREBY CREATES A MORE EFFICIENT AN SHORTER OVERALL TIME FOR THE STERILIZING CYCLE. A CONTROL IS USED, MADE UP OF A NEEDLE VALVE IN THE STEAM LINE WHICH IS USED TO SET THE APPROXIMATE FLOW RATE. ACTUALLY, THIS FLOW RATE IS SLIGHTLY GREATER THAN THE FLOW RATE REQUIRED. AN ADDITIONAL SOLENOID VALVE IS PLACED IN THE STEAM LINE IN SERIES WITH THE NEEDLE VALVE AND A THERMOSTATIC CONTROL IS PLACED IN THE CHAMBER DRAIN LINE TO TURN THE SOLENOID VALVE ON AND OFF TO MAINTAIN THE DESIRED LOAD TEMPERATURE AT APPROXIMATELY 130*F. THE TURNING ON AND OFF OF THE SOLENOID VALVE TO MODULATE THE DRAIN LINE TEMPERATURE SETS UP A PLUSING FLOW OF STEAM AND STEAM PRESSURE CONDITION OF APPROXIMATELY 25 TO 30 IN. HG WHICH HELPS TO PURGE THE LOAD OF ENTRAINED AIR THEREBY HEATING THE LOAD MORE RAPIDLY. FOLLOWING THE SIMULTANEOUS APPLICATION OF VACUUM AND STEAM, A MICROBIOCIDAL CHEMICAL STERILIZING GAS IS ADMITTED TO THE STERILIZING CHAMBER.

Aug. 10, 1971 J. J. SHULL ETAL 3,598,516

METHOD OF STERILIZING Filed Jan. 2, 1969 2 Sheets-Sheet 1 l0 ETHYLENEOXIDE STEAM /MO!STURE I HR 5! I l 8? TEMQ l ATMOS w g I PRESSuRE V) I 51Q 0 o n. MIN. TIME 70 E Fig. 2

DR JAMES J. SHULL ROBERT S. LLOYD United States Patent US. CI. 2157 5Claims ABSTRACT OF THE DISCLOSURE The sterilizing process is carried outin a sealed container or autoclave wherein air in the chamber isdisplaced with steam while the chamber is maintained at vacuumcondition. The steam is fed in at a rate no greater than the capacity ofthe vacuum pump so that the pump will maintain the desired pressure inthe chamber. Steam thereby heats the load to the desired temperature ina short time and thereby creates a more eflicient and shorter over alltime for the sterilizing cycle. A control is used, made up of a needlevalve in the steam line which is used to set the approximate flow rate.Actually, this flow rate is slightly greater than the flow raterequired. An additional solenoid valve is placed in the steam line inseries with the needle valve and a thermostatic control is placed in thechamber drain line to turn the solenoid valve on and off to maintain thedesired load temperature at approximately 130 F.

The turning on and off of the solenoid valve to modulate the drain linetemperature sets up a pulsing flow of steam and steam pressure conditionof approximately 25 to 30 in. Hg which helps to purge the load ofentrained air thereby heating the load more rapidly. Following thesimultaneous application of vacuum and steam, a microbiocidal chemicalsterilizing gas is admitted to the sterilizing chamber.

CROSS REFERENCE TO RELATED APPLICATIONS This application is acontinuation-in-part of patent application, Ser. No. 414,721, filed Nov.30, 1964 and now abandoned.

FIELD OF INVENTION This invention relates to sterilizing methods and,more particularly, to a method of sterilizing with a gaseous agent,requiring moisture for its activity whereby said method also insuresrapid heating as well as moisturization of materials to be sterilized,thereby permitting shorter sterilizing times than have heretofore beenpossible in commercially available equipment.

DESCRIPTION OF PRIOR ART The Merriam Pat. 2,080,179 discloses asterilizing cycle wherein steam is introduced to the chamber after thechamber has been evacuated and prior to the introduction of gas. Merriamsuggests dry steam at atmospheric temperature and suggests that theproduct may be heated before putting it into the chamber.

The Hickey Pat. 3,035,886 discloses a sterilizing process wherein theair from the chamber is purged with steam at atmospheric pressure andthereafter, ethylene oxide gas is introduced.

SUMMARY OF INVENTION It is the purpose of this invention to provide amethod and apparatus whereby the sterilizing cycle time is substantiallyshorter than conventional cycles now in use, and whereby the protectiveeffect on the bacteria by contaminating debris or salts is minimized orovercome.

3,598,516 Patented Aug. 10, 1971 It is another object of the inventionto limit the necessity for humidity measuring and control systems on gassterilizers.

These objects are accomplished by two unique innovations in the methodof heating and moisturizing the material within the sterilizer chamber.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a diagrammatic view of thesterilizer chamber and functioning components according to theinvention.

FIG. 2 is a graph showing load temperature, chamber relative humidityand chamber pressure during the course of a typical sterilizing cycleaccording to the invention.

FIG. 3 is a diagram which shows the advantage of time saving over aknown sterilizing cycle.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT It is known in the art thatthe time required to sterilize with ethylene oxide is inverselyproportional to the temperature of the most remote part of the materialsto be sterilized with ethylene oxide. Laboratory investigations revealedthat injection of steam into the chamber at low absolute pressures wasthe most rapid way practical to heat materials contained within saidchamber. It was discovered that to carry out such a process mostefficiently a chamber containing materials to be sterilized may beevacuated and steam may be injected into the chamber simultaneously withthe evacuation. Such operation may be continued for a sufficient time toallow the steam to displace substantially all air from the materials inthe chamber and to establish the desired temperature in the goods.

Because steam moisturizes materials upon which it condenses during theheating process, near complete humidification of the materials results.Both the heating and the humidification process, however, are dependentupon substantially complete elimination of air from the chamber and thematerials contained therein. Experimental studies showed that airadmitted to the sterilizer together with the injected steam at lowpressure interfered with sterilization of bacterial spore control.

When the vacuum-steam portion of the cycle has been continued for asuificient time for the materials to achieve a suitable temperature, forexample, 130 F. plus or minus 5 F. throughout the materials beingpreferred, the vacuum line is closed and the sterilizing gas is admittedto the chamber until a suitable gas concentration is achieved. The gasadmission is then terminated, gas concentration being maintained,however, by adding make up gas to the chamber as required. Experimentshave shown that steam, together with the gas in addition to thehumidifying initially, was advantageous to obtain a sterility of sporetest preparations in which the spores were protected by salts orcellular and organic materials. It is therefore desirable to add steamduring gas change.

In carrying out the invention, a sterilizing chamber 10 was outfittedwith a vacuum pump 12 connected to the drain line 13 in the bottom ofthe chamber. A heat exchanger 14 was provided and steam at, for example,50 to p.s.i.g. connected as indicated. To heat, steam is fed to thesterilizer chamber through the charging line 5. Steam was admitted tothe chamber through the heat exchanger through the metering valve 17 andthe solenoid valve 16. The ethylene oxide was admitted through valve 18and through the heat exchanger 14 to the sterilizer 10. The walls of thesterilizer chamber were preheated by a steam jacket (not shown) to thedesired temperature.

A thermostat 23 in the drain line controlled the solenoid valve in thesteam line so that the temperature of the drain line remained atapproximately F. The metering valve 17 was set to give a flow slightlyabove that required to maintain the desired temperature in the chamber,the proper amount of steam being controlled by solenoid valve 16actuated by thermostat 23. The gas may be flushed from the chamber byfiltered air by way of valve 21 and filter 22. All pressures referred toin this specification are absolute pressures unless otherwise stated.

The following are examples of operating cycles:

EXAMPLE I 1) Load the chamber with materials to be sterilized.

(2) Start vacuum pump 12.

(3) Open valve 19 and valve 17 and valve 16 to admit steam through theheat exchanger 14 at a rate that is 'slightly greater than thatnecessary to maintain the temperature of approximately 130 F. in theload.

Continue evacuating and injecting steam for a period of time necessaryfor the temperature of the load to reach 130 F. throughout.

(4) Close valve 19 and open valve 18 to admit sterilizing gas leavingvalve 16 open to admit sterilizing gas and steam simultaneously tochamber 10. Continue admission of gas and steam until the chamberpressure reaches 8 p.s.i.g.

(5) Close valve 17 and valve 18. Maintain pressure for one half hour byadding ethylene oxide mixture as necessary through valve 18.

(6) Open valve 19 and evacuate chamber to 26 inches Hg.

(7) Close valve 19 and open valve 21 to admit sterile air through filter22.

Employing the method and apparatus just described, the cycle timerequired to sterilize materials and bacterial spores Within the chamberwas reduced to less than 60 minutes from the 4 to 6 hour cycle common toconventional systems. Furthermore, this improved method is capable ofdestroying spores encapsulated within salt crystals and embedded inorganic materials within 1 or 2 hours as opposed to the inability ofconventional systems to sterilize such preparations regardless of timeemployed within reasonable limits.

EXAMPLE II The chamber was loaded with a cardboard carton containing asmany cardboard inserts as the carton would hold with sterilitypredicated upon the sterilization of a number of accepted sterilitycontols inserted in the load. The cardboard carton was 12 x 12 x 22inches in dimension. Thermocouple lead wires were connected to apotentiometer for measuring temperature rise within the cardboard carbonplaced in various locations. A humidity sensing element for measuringthe moisture content of the load was inserted in the cardboard carton.

Sterility controls consisting of glassine envelopes containing strips offilter paper impregnated with bacterial spores and dried at standardtemperatures and pressures and spore-containing ceramic tiles, alsopacked in glassine envelopes, were placed in select locations inside thecardboard cartons. After placing the prepared and sealed cardboardcartons into the sterilizing chamber and activating the temperaturerecording potentiometer and humidity detecting equipment, the gas cycleas described above was initiated, using the following steps:

(1) Evacuate the chamber to 20 in. Hg and introduce steam during thetime of evacuation at a rate equivalent to that required to maintain aload temperature of 130 F. with the vacuum system in operation.

(2) Maintain these conditions for a period of 20 minutes.

(3) Introduce an ethylene oxide-Freon gas sterilizing mixture to achamber pressure of 8 p.s.i.g. at a rate of .50 lb./minute.

(4) Expose material load at 8 p.s.i.g. for a period of 60 minutes.

(5) Exhaust chamber to atmosphere and evacuate to 26 in. Hg and flushchamber with filtered air to atmospheric pressure.

(6) Repeat step 5.

7) Remove material load and test sterility controls for sterility usingaccepted bacteriological techniques.

Results: Sterility results were satisfactory.

EXAMPLE III The chamber was loaded with packaged articles representativeof those commonly used in a typical hospital load as predicated upon thesterilization of a number of accepted sterility controls inserted in thepackaged articles and using the gas cycle as described herein.

In this example, the steps were exactly as in Example II with theexception that the exposure period was 35 minutes.

Results: Load temperatures, humidity readings and chamber temperatureswere comparable and closely similar to those illustrated in Example II.Sterility results were satisfactory.

EXAMPLE IV A typical load of hospital packaged articles with sterilitypredicated upon the sterilization of accepted sterility controlsinserted in the packaged articles was placed in the chamber. Thematerials were made up of hospital articles consisting of packagedrubber cathethers, packaged anesthesia equipment and packaged surgicaldressings placed in metal trays to simulate a typical hospital load.

The steps were carried out as in Examples II and III. The exposureperiod used was 40 minutes.

Results: The load temperature, humidity and chamber temperatures wereclosely comparable to those in the prior examples. Sterility resultswere satisfactory.

EXAMPLE V A load of packaged hospital materials typical of materialsused in hospitals and predicating sterility upon the sterilization ofaccepted sterility controls inserted in the packaged articles and usingthe gas cycle as described in Example II.

Materials consisted of such articles as packaged plastic boxes, suctioncatheters, plastic tubes and items of small glassware were used to makeup the test load.

The cycle was carried out as described in Example II allowing forexposure time of 45 minutes.

Results: The results were satisfactory.

EXAMPLES OF TESTS WHEREIN THE RESULTS WERE UNSATISFACTORY That is,non-sterility of the load resulted.

In contrast to the tests described above and to illustrate theineffectiveness of gas cycles which are somewhat similar to thatdescribed but which differ in certain preconditioning techniques, thefollowing examples are presented. To illustrate the manner in which thegas cycles differ from those of the original cycle, a comparison of thecycle variations with the patent application cycle was made as follows:

EXAMPLE I(A) Comparison of cycles A chamber was loaded with a number ofsterility controls comprising spore contaminated strips of filter paperpackaged in glassine, paper, muslin and plastic film envelopes, glassbeads contaminated with water, saline and blood serum spore suspensions,and also packaged in similar envelopes and cotton-plugged test tubescontaining spore strips and spore contaminated glass beads were placedin the material load comprising 6 cardboard cartons with cardboardinserts to illustrate a full hospital load of porous materials. The loadwas placed in the sterilizer and exposed to the inoperable cycle for aperiod of 15 minutes.

Patent application cycle Cycle variation Results satisfactory Resultsunsatisfactory EXAMPLE II (A) Cycle variation Patent application cycle(a) Chamber vacuum with steam (a) Chamber vacuum with steam injectionfor 20 minutes (preinjection for minutes. conditioning (b) Gas chargewith steam to 8.0 (b) Gas charge with steam to 8.0

p.s.i.g. in 2-3.0 minutes. p.s.i.g. in 2-3.0 min.

(0) Exposure period of 60 minutes (0) Exposure period of 30 minutes.

((1) Cliamlggr temperature: 130 ((1) Cliamg er temperature: 130

Results satisfactory Results unsatisfactory Material load.A materialload was prepared for this test comprising three cardboard cartons (9 /2x 9 /2 x /2 inches) filled with cardboard inserts.

Sterility test results are shown in Example II(A).

EXAMPLE III(A) Patent application cycle Cycle variation (a) Chambervacuum with steam (a) Chamber vacuum with no injection for minutes(presteam injection. conditioning).

(b) Gas charge with steam to 8.0 (b) Gas charge with steam to 8.0

p.s.i.g. in 2-3.0 minutes. p.s.i.g. in 2-3.0 min.

(0) Exposure period of 60 minutes (0) Exposure period of 60 min.

(d) Cgflmbgl temperature: 130 (d) Cllr amg er temperature: 130

Results satisfactory Results unsatisfactory Material load-The materialload for this test was identical to that described for Example 11.

Sterility controls.-The sterility controls were identical to thoselisted for Example II and were inserted inside the three cardboardcartons in approximately the same areas as those employed in Example II.

EXAMPLE IV(A) Cycle variation (unsuccessful cycle) Patent applicationcycle (successful cycle) (:1) Chamber vacuum with steam (a) Chambervacuum with steam injection for 20 minutes (preinjection ior 10 minutes.conditioning).

(b) Gas charge with steam to 8.0 (b) Continuation of vacuum withp.s.i.g. in 2-3.0 minutes. steam air mixture for 10 minutes.

(0) Exposure period of 60 minutes (0) Gas charge with steam to 8.0

p.s.i.g. in 2-3.0 min.

(d) Cliambsr temperature: 130 (d) Exposure period of 60 min.

(e) Chamber temperature: 130

Results satisfactory Results unsatisfactory EXAMPLE V(A) Cycle variationPatent application cycle (a) Chamber vacuum with steam (a) Chambervacuumsteam injection for 20 minutes (preinjection for 8.0 min.conditioning) 0)) Gas charge with steam to 8.0 (b) Continuation ofchamber p.s.i.g. in 2-3.0 minutes. vacuum with steam and air injectionfor 10 min.

(c) Exposure period of 60 minutes (0) Gigs charge to 8.0 p.s.i.g. in

3.0 minutes. ((1) Chamber temperature: 120 F. (d) Exposure period of 120minutes (e) Chamber temperature: 130

Material load.The material load for this test was identical to thatdescribed for Example II.

Sterility control.The sterility controls were similar to those listedfor Example II and were inserted inside the three cardboard cartons inapproximately the same areas as those employed in Example H.

Results.Sterility of the sterility controls was not achieved in thistest due to the irregular conditions employed in the preconditioningphase.

EXAMPLE VI(A) Patent application cycle Cycle variation (a) Chambervacuum with injec- (a) Chamber vacuum with steam gon for)20 minutes(precondiinjection for 8 minutes.

oning (b) Gas charge with steam to 8.0 (0) Gas charge to 8.0 p.s.i.g. in

p.s.i.g. in 2-3.0 minutes. 2-3.0 mi

(c) Exposure period of 60 minutes" (c) Exposure period of 30 minutes.

((1) Chamber temperature: ((1) Chamber temperature Results satisfactoryResults unsatisfactory Material Load.-A material load was prepared forthis test comprising a typical hospital load of a variety of typicalhospital items such as surgical instruments, face masks, rubber gloves,and catheters, wrapped in paper and polyethylene film (2.0 milsthickness).

CONCLUSIONS It was concluded that the overall results of the studiesconducted showed that a properly synchronized preconditioning period wasnecessary in order to achieve sterility of the load materials. This isillustrated by the manner in which a controlled flow of steam isintroduced into the chamber during the initial vacuum resulting in themaintenance of a specific range of absolute pressure within the chamberand the continuous flow of a time period sufiicient for the goods toequilibrate in temperature and become moisturized.

The attached examples show that under these conditions, the materialload is heated and moisturized to a condition in which sterility isattained within a period shorter than many of the presently usedethylene oxide cycles.

In contrast, when the preconditioning factors are changed or modified tothe extent that the desired chamber absolute pressure is not attainedor, if attained, is not maintained for a sufficient period of timeresulting in an unheated condition of the goods and non-moisturizedgoods, sterility is not achieved even though the actual exposure timesof the material load to the sterilizing agent are exactly the same.

Graph lines, FIG. 3

FIG. 3 shows the relationship of load temperatures occurring in a gascycle utilizing the steam preconditioning phase described herein and ina gas cycle where the preconditioning phase is Omitted.

Line ACD shows a pressure-time curve in a sterilizing cycle where gas isadmitted immediately after vacuum is established. Line HI is a loadtemperature curve which occurs under the conditions designated by lineACD. Line AKI shows a pressure-time curve in a sterilizing cycle, suchas described in the present disclosure; in which steam is introduced forconditioning at point B. Line EFG is a temperature-time curve whichoccurs wherein steam is injected at the beginning of the Nacuum orconditioning cycle in accordance with the method disclosed herein. Thisline shows that the load being sterilized has reached the desiredoperating temperature prior to the time the gas is admitted at point K.

The graph shows that in the cycle disclosed herein the load temperatereaches the desired value throughout in about 10 minutes. In the sametime, using a cycle in which steam is not admitted according to thedisclosed method, the load temperature had reached only about 27 C. (80F.) at the time gas was admitted. This temperature is far below thedesired operating temperature.

Since gas sterilization takes place at a very slow rate at 27 C. (80F.), this particular cycle had to be a great deal longer in time thanthe cycle disclosed.

By contrast, with a cycle shown by curve EFG, the temperature of thecenter of the load had reached the desired 54 C. (130 F.) in a muchshorter time thus allowing for a much shorter sterilizing time. Thus, itis clear that with the cycle Where steam is injected during the time thechamber is being evacuated, the load temperature reaches the desired 54C. (130 F.) before gas admission. Therefore, FIG. 3 shows that the totalcycle time with steam injection in accordance with the examplesdisclosed herein will be shorter than with prior cycles.

In summary, rise of load temperature is very slow, particularly when thepreconditioning cycle is omitted and the ethylene oxide mixture isintroduced into the chamber immediately after the desired initial vacuumis obtained. Since it has been established that the load of materialsis, no doubt, moisturized during the preconditioning phase, although theactual concentration of moisture is not intentionally measured or evenmaintained at a selected level, humidity curves have been omitted fromthe graph. As indicated in FIG. 3, load temperatures occurring duringthe disclosed cycle using no conditioning are very slow in rising to thedesired operating temperature, 54C. (130 F.). Significant temperaturerise occurs only after the chamber has been charged and the exposureperiod has been initiated. This is in direct contrast to the shown inthe curve EFG where load temperatures rise during the steam-vacuumconditioning phase and reach the desired operating temperature beforethe chamber is charged with gas.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A method of sterilizing comprising loading a sealed chamber witharticles to be sterilized,

applying a vacuum means to said chamber thereby reducing the pressurebelow atmospheric and simultaneously admitting steam to said chamber ata rate that will maintain the temperature of said chamber at a desiredtemperature, and continuing said simultaneous vacuum application andsteam admission for a predetermined period,

and stopping said vacuum means and stopping said steam means to saidchamber and admitting a microbiocidal chemical sterilizing gas to saidchamber, and maintaining said sterilizing gas in said chamber for apredetermined time period for sterilization of the articles disposedtherein. 2. The method recited in claim 1 wherein said vacuum meansmaintains said chamber at approximately 20 inches Hg absolute.

3. The method recited in claim 1 wherein said sterilizing gas isethylene oxide and said sterilizing gas is admitted at the rate ofapproximately 0.5 pound per minute.

4. The method recited in claim 3 wherein said steam and said vacuum aremaintained in said chamber for a period as determined by the size of theload to ensure maintenance of said desired temperature. 5. A method ofsterilizing comprising loading a sealed chamber with articles to besterilized, applying a vacuum means to said chamber thereby reducing thepressure in the chamber below atmospheric and simultaneously admittingsteam to said chamber at a rate that will maintain the temperature ofsaid chamber at a desired temperature, and continuing said simultaneousvacuum application and steam admission for a predetermined period,

and stopping said vacuum means and stopping said steam means to saidchamber and admitting a mixture of microbiocidal chemical sterilizinggas and steam to said chamber to bring the pressure in said chamber to adesired pressure and maintaining said mixture of sterilizing gas andsteam in said chambe for a predetermined time period for sterilization athe articles disposed therein.

References Cited UNITED STATES PATENTS 2,080,179 5/1937 Merriam et al.21DIG.-4 3,035,886 5/1962 Hickey 21DIG.4 3,042,533 7/1962 McConnell etal. 2l-DIG.-4 3,068,064 12/1962 McDonald 2l-DIG.4 3,372,980 3/1968 Satas2lDIG.-4 3,409,389 11/1968 Bjork 21--56 FOREIGN PATENTS 656,699 1/ 1963Canada 21-DIG.-4

1,148,704 5/1963 Germany 21-DIG.4

JOSEPH SCOVRONEK, Primary Examiner B. S. RICHMAN, Assistant Examiner US.Cl. X.R. 21--58, DIG.4

